No matter how many jokes she makes about it, nor how often she raises an ironic but perfectly plucked eyebrow in the direction of the TV lights, the story of Carrie Fisher is pure, distilled Hollywood - one heady swig and you're caught in the middle of a Technicolor collage of drink and drugs, sex and celebrity. When Daddy (Eddie Fisher) left poor Mommie (Debbie Reynolds) for the actress with the violet eyes (Elizabeth Taylor), it was as if a strange prophecy rang out across the city of swimming pools and studio lots. 'Thou shalt screw up royally,' boomed the voice, in sombre, movie-trailer tones. 'But this will not matter too much because, as every writer knows, mistakes are just good material wrapped in a damp handkerchief.'

And so it proved. First, Fisher became famous in her own right, the better that her slip-ups might be marketable (even now, I think she looks odd without the two bagels she wore on her head when she played Princess Leia). Then she got on with creating mayhem. There was booze (she once put away so much that she had to ask John Belushi to punch her in the stomach so she could be sick before she went to bed rather than after); there were pills (so many that, on the move, she must have sounded like an old yoghurt pot filled with dried peas); and there were relationship crises (her marriage to Paul Simon lasted 11 months, and the father of her daughter left her for a man). Along the way, however, she obviously took plenty of notes. Is there no part of her life that she is not prepared to dish up in her novels? In a word, no.

The heroine of the bafflingly titled The Best Awful is Suzanne Vale, who made her fictional debut in Fisher's first book, Postcards From the Edge. Since Postcards, Vale has been on the straight and narrow - or at least her version of it. A successful chat-show host, albeit on an obscure cable channel, these days her only vice is the odd can of Diet Coke; she is, she tells us, 'a breadwinner with a very yang personality'.

Then again, life is hardly one big Slim Aarons photograph. OK, she has a pool out back, and is the lucky recipient of invitations to all the best Hollywood memorial services. But none of that means terribly much when your husband - yes, the father of her daughter - has run off with a deliciously clean-looking man, and you are left with only your bipolar medication for company.

To prove, once and for all, that she does know a straight man from a gay one, Suzanne decides to get thoroughly laid. This she does, with some aplomb, at one of the aforementioned memorial services (her prey is Dean Bradbury, a wolfish Jack Nicholson figure), thus ending her 'four-year sex drought with a really big box-office bang'. Encouraged, next up is Thor, a Serbian hunk 20 years her junior. Thor, however, is swiftly dispatched when she begins to fear she is making a fool of herself.

Third in line, she decrees, will be Mark Vogel, the producer of a hit TV show. But when she makes a pass at him, he rebuffs her. It is at this point that she decides, quite inexplicably, to stop taking the pills that so trammel her personality, damping down the 'Lucretia' figure who lurks within. 'One more dance in the light,' she thinks. And then? She will embrace a docile middle age.

Hereafter, all hell breaks loose. In fictional terms, there are few things so boring as a nervous breakdown (unless we are talking Salinger), and the one described in such breathless detail in The Best Awful ranks with the work of Elizabeth Wurtzel when it comes to tedium. On a manic high, Vale hits on her tattoo artist, accepts his offer of a shedload of a prescription painkiller called OxyContin, and then drives with him to Tijuana in pursuit of yet more of the stuff. There, she gets scared, vomits copiously and rings her best friend who comes to rescue her.

Back in LA, she takes an overdose and is sent to a psychiatric clinic. Alas, during all this pain and narcissism, we are presented with just one lonely glimmer of writerly insight. 'She felt as though she had been dropped into the centre of her personality years ago and had been trying to crawl to the edge of it ever since,' we are told, as Suzanne bobs to the surface of consciousness.

The Best Awful, whose unlikely Hollywood ending I won't spoil, suffers from a literary version of the bipolar disorder that so cripples its heroine (and - surprise - its author, who requires two dozen pills a day to control the condition). When it is good, it is... well... fine; when it is bad, which is too often, it is absolutely bloody awful, so toxically terrible you feel like rushing out of doors and burying it at the bottom of the garden.

Now and then, Fisher can spook up a perfectly serviceable image. Examining her post-baby belly, Suzanne notices the fat clinging to her like 'frightened tenants in dark houses when the ghosts come out'. Mostly, though, she writes like a crazed rambler, heaping word upon word until her sentences resemble great, wobbly cairns; examine them too closely and they topple like nine-pins. There are so many reasons for adoring Fisher - the camp private life, the irrepressible spirit, those bagels! - but this book, sad to say, is simply not one of them.